1. Field of the Invention
This invention relates to circuit breakers having a trip unit, and more specifically, to a lock out assembly for controlling access to the trip unit.
2. Background Information
Circuit breakers, and in particular, circuit breakers of the molded case variety (see, for example, U.S. Pat. No. 5,341,191), are generally known in the art and used for protecting electrical circuitry from damage due to an overcurrent condition, such as an overload condition or a relatively high level short circuit or fault condition. Molded case circuit breakers typically include a pair of separable contacts per phase which may be operated either manually by way of a handle disposed on the outside of the case or automatically in response to an overcurrent condition. Typically, such circuit breakers include an operating mechanism, which is designed to rapidly open and close the separable contacts, and a trip unit, which senses overcurrent conditions in an automatic mode of operation. Upon sensing an overcurrent condition, the trip unit trips the operating mechanism to a trip state which moves the separable contacts to their open position.
It is also well known to employ trip units which utilize a microprocessor to detect various types of overcurrent trip conditions, such as, for example, a long delay trip, a short delay trip, an instantaneous trip, or a ground fault trip. The long delay trip function protects the load served by the protected electrical system from overloads and/or overcurrents. The short delay trip function can be used to coordinate tripping of down stream circuit breakers in a hierarchy of circuit breakers. The instantaneous trip function protects the electrical conductors to which the circuit breaker is connected from damaging overcurrent conditions, such as short circuits. As implied, the ground fault trip function protects the electrical system from faults to ground.
Each circuit breaker is designed for a specific maximum continuous current. This current rating may be set by selection of a resistor which converts the current to a voltage for use by the trip unit. This device is commonly referred to in the art as a "rating plug." In some instances, a single circuit breaker frame may be easily adapted for installations which call for a range of maximum continuous currents, up to the design limits of the frame, through use of the rating plug by which the current rating of the device can be established. Typically, the pick-up currents for the various protection functions have been selectable multiples or fractions of this current rating. Thus, instantaneous protection trips the device any time the current reaches a selected multiple of the rated current, such as for example, ten times the rated current. Pick-up for short delay protection is a lesser multiple of the rated current, while pickup current for long delay protection may be a fraction of the rated current.
The long delay and short delay trip functions require that the overcurrent condition exists for a period of time. Typically, the long delay trip function has been selected such that a trip signal is generated if the current exceeds six times the rated current for the selected long delay interval. As damage can also occur at lesser current levels for longer periods of time, an inverse time function is used for long delay protection. Thus, the smaller the current the longer the time to trip, and the larger the current, the shorter the time to trip.
With the traditional approach of having the instantaneous, short delay and long delay pick-up currents selectable as factors of the rated current, the protection functions are independent of one another, so that setting the parameters for one function does not typically affect the setting of parameters for another function. Typically, setting of the parameters for each of the protection functions is achieved by a user interfacing with the various settings on the trip unit. For example, it is known to provide adjustment and setting means that are recessed from the face of the trip unit and adjustable by an appropriate tool, such as a screwdriver. In this type of arrangement, the settings can be easily changed without proper authorization. This can cause nuisance trip situations due to the incorrect settings of the circuit breaker, or cause damage to the load.
In addition, it is typically the situation where the rating plug may also be reset, or removed altogether, due to its positioning on or adjacent the face of the trip unit, which is typically located on the outside of the circuit breaker housing. This also can result in improper protection to the electrical system and in nuisance trip situations, as well as, damage being done to the circuit breaker due to improper rating plugs or settings thereof being employed.
There is a need, therefore, for a circuit breaker having a trip unit that prevents adjustments being made to the settings for the operating parameters and/or the rating plug and settings thereof without proper authorization.